Methods for improving transmission efficiency of control channels in communication systems

ABSTRACT

Embodiments of the invention provide methods for optimizing the spectral efficiency of control channel transmissions carrying scheduling assignments from a serving Node B to user equipments. This is accomplished through the selection of the modulation and coding scheme with the highest spectral efficiency that can support transmission of a scheduling assignment to the corresponding user equipment with a reception performance satisfying a target error rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and incorporates by reference UnitedStates Provisional Application Nos.: 60/732,868, filed Nov. 2, 2005;60/746,450 filed May 4, 2006; and 60/805,148 filed Jun. 19, 2006.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Embodiments of the invention are directed, in general, to communicationsystems and, more specifically, to optimizing the spectral efficiency ofthe transmission of control channels in the downlink of a communicationsystem.

The global market for both voice and data communication servicescontinues to grow as does use of the systems which deliver suchservices. As communication systems evolve, system design has becomeincreasingly demanding in relation to equipment and performancerequirements. Future generations of communication systems, will berequired to provide high quality high transmission rate data services inaddition to high quality voice services. Orthogonal Frequency DivisionMultiplexing (OFDM) is a technique that will allow for high speed voiceand data communication services.

Orthogonal Frequency Division Multiplexing (OFDM) is based on thewell-known technique of Frequency Division Multiplexing (FDM). OFDMtechnique relies on the orthogonality properties of the fast Fouriertransform (FFT) and the inverse fast Fourier transform (IFFT) toeliminate interference between carriers. At the transmitter, the precisesetting of the carrier frequencies is performed by the IFFT. The data isencoded into constellation points by multiple (one for each carrier)constellation encoders. The complex values of the constellation encoderoutputs are the inputs to the IFFT. For wireless transmission, theoutputs of the IFFT are converted to an analog waveform, up-converted toa radio frequency, amplified, and transmitted. At the receiver, thereverse process is performed. The received signal (input signal) isamplified, down converted to a band suitable for analog to digitalconversion, digitized, and processed by a FFT to recover the carriers.The multiple carriers are then demodulated in multiple constellationdecoders (one for each carrier), recovering the original data. Since anIFFT is used to combine the carriers at the transmitter and acorresponding FFT is used to separate the carriers at the receiver, theprocess has potentially zero inter-carrier interference such as when thesub-carriers are separated in frequency by an amount larger than themaximum expected Doppler shift.

FIG. 1 is a diagram illustrative of the Frequency 103-Time 101Representation 100 of an OFDM Signal. In FDM different streams ofinformation are mapped onto separate parallel frequency channels 140.Each FDM channel is separated from the others by a frequency guard bandto reduce interference between adjacent channels.

The OFDM technique differs from traditional FDM in the followinginterrelated ways:

-   -   1. multiple carriers (called sub-carriers 150) carry the        information stream;    -   2. the sub-carriers 150 are orthogonal to each other; and    -   3. a Cyclic Prefix (CP) 110 (also known as guard interval) is        added to each symbol 120 to combat the channel delay spread and        avoid OFDM inter-symbol interference (ISI).

The data/information carried by each sub-carrier 150 may be user data ofmany forms, including text, voice, video, and the like. In addition, thedata includes control data, a particular type of which is discussedbelow. As a result of the orthogonality, ideally each receiving elementtuned to a given sub-carrier does not perceive any of the signalscommunicated at any other of the sub-carriers. Given this aspect,various benefits arise. For example, OFDM is able to use orthogonalsub-carriers and, as a result, thorough use is made of the overall OFDMspectrum. As another example, in many wireless systems, the sametransmitted signal arrives at the receiver at different times havingtraveled different lengths due to reflections in the channel between thetransmitter and receiver. Each different arrival of the sameoriginally-transmitted signal is typically referred to as a multi-path.Typically, multi-paths interfere with one another, which is sometimesreferred to as InterSymbol Interference (ISI) because each path includestransmitted data referred to as symbols. Nonetheless, the orthogonalityimplemented by OFDM with a CP considerably reduces or eliminates ISIand, as a result, a less complex receiver structure, such as one withoutan equalizer (one-tap “equalizer” is used), may be implemented in anOFDM system.

The Cyclic Prefix (CP) (also referred to as guard interval) is added toeach symbol to combat the channel delay spread and avoid ISI. FIG. 2 isa diagram illustrative of using CP to eliminate ISI and performfrequency domain equalization. Blocks 200 each comprising cyclic prefix(CP) 210 coupled to data symbols 220 to perform frequency domainequalization. OFDM typically allows the application of simple, 1-tap,frequency domain equalization (FDE) through the use of a CP 210 at everyFFT processing block 200 to suppress multi-path interference. Two blocksare shown for drawing convenience. CP 210 eliminates inter-data-blockinterference and multi-access interference using Frequency DivisionMultiple Access (FDMA).

Since orthogonality is typically guaranteed between overlappingsub-carriers and between consecutive OFDM symbols in the presence oftime/frequency dispersive channels, the data symbol density in thetime-frequency plane can be maximized and high data rates can be veryefficiently achieved for high Signal-to-Interference and Noise Ratios(SINR).

FIG. 3 is a diagram illustrative of CP Insertion. A number of samplesare typically inserted between useful OFDM symbols 320 (guard interval)to combat OFDM ISI induced by channel dispersion, assist receiversynchronization, and aid spectral shaping. The guard interval 310 istypically a prefix that is inserted 350 at the beginning of the usefulOFDM symbol (OFDM symbol without the CP) 320. The CP duration 315 shouldbe sufficient to cover most of the delay-spread energy of a radiochannel impulse response. It should also be as small as possible sinceit represents overhead and reduces OFDM efficiency. Prefix 310 isgenerated using a last block of samples 340 from the useful OFDM symbol330 and is therefore a cyclic extension to the OFDM symbol (cyclicprefix).

When the channel delay spread exceeds the CP duration 315, the energycontained in the ISI should be much smaller than the useful OFDM symbolenergy and therefore, the OFDM symbol duration 325 should be much largerthan the channel delay spread. However, the OFDM symbol duration 325should be smaller than the minimum channel coherence time in order tomaintain the OFDM ability to combat fast temporal fading. Otherwise, thechannel may not always be constant over the OFDM symbol and this mayresult in inter-sub-carrier orthogonality loss in fast fading channels.Since the channel coherence time is inversely proportional to themaximum Doppler shift (time-frequency duality), this implies that thesymbol duration should be much smaller than the inverse of the maximumDoppler shift.

The large number of OFDM sub-carriers makes the bandwidth of individualsub-carriers small relative to the total signal bandwidth. With anadequate number of sub-carriers, the inter-carrier spacing is muchnarrower than the channel coherence bandwidth. Since the channelcoherence bandwidth is inversely proportional to the channel delayspread, the sub-carrier separation is generally designed to be muchsmaller that the inverse of the channel coherence time. Then, the fadingon each sub-carrier appears flat in frequency and this enables 1-tapfrequency equalization, use of high order modulation, and effectiveutilization of multiple transmitter and receiver antenna techniques suchas Multiple Input/Multiple Output (MIMO). Therefore, OFDM effectivelyconverts a frequency-selective channel into a parallel collection offrequency flat sub-channels and enables a very simple receiver.Moreover, in order to combat Doppler effects, the inter-carrier spacingshould be much larger than the maximum Doppler shift.

FIG. 4 shows the concepts of frequency diversity 400 and multi-userdiversity 405. Using link adaptation techniques based on the estimateddynamic channel properties, the OFDM transmitter can adapt thetransmitted signal to each User Equipment (UE) to match channelconditions and approach the ideal capacity of frequency-selectivechannel. Thanks to such properties as flattened channel per sub-carrier,high-order modulation, orthogonal sub-carriers, and MIMO, it is possibleto improve spectrum utilization and increase achievable peak data ratein OFDM system. Also, OFDM can provide scalability for various channelbandwidths (i.e. 1.25, 2.5, 5, 10, 20 MHz) without significantlyincreasing complexity.

OFDM may be combined with Frequency Division Multiple Access (FDMA) inan Orthogonal Frequency Division Multiple Access (OFDMA) system to allowmultiplexing of multiple UEs over the available bandwidth. Because OFDMAassigns UEs to isolated frequency sub-carriers, intra-cell interferencemay be avoided and high data rate may be achieved. The base station (orNode B) scheduler assigns physical channels based on Channel QualityIndication (CQI) feedback information from the UEs, thus effectivelycontrolling the multiple-access mechanism in the cell. For example, inFIG. 4, transmission to each of the three UEs 401, 402, 403 is scheduledat frequency sub-bands where the channel frequency response allows forhigher SINR relative to other sub-bands. This is represented by theReceived signal levels R401, R402, and R403 for users 401, 402 and 403at Frequencies F401, F402, and F403 respectively.

OFDM can use frequency-dependent scheduling with optimal per sub-bandModulation & Coding Scheme (MCS) selection. For each UE and eachTransmission Time Interval (TTI), the Node B scheduler selects fortransmission with the appropriate MCS a group of the active UEs in thecell, according to some criteria that typically incorporate theachievable SINR per sub-band based on the CQI feedback. A UE may beassigned the same sub-band for transmission or reception of its datasignal during the entire TTI. In addition, sub-carriers or group ofsub-carriers may be reserved to transmit pilot, control signaling orother channels. Multiplexing may also be performed in the timedimension, as long as it occurs at the OFDM symbol rate or at a multipleof the symbol rate (i.e. from one TTI to the next). The MCS used foreach sub-carrier or group of sub-carriers can also be changed at thecorresponding rate, keeping the computational simplicity of theFFT-based implementation. This allows 2-dimensional time-frequencymultiplexing, as shown in FIG. 5 and FIG. 6.

Turning now to FIG. 5, which is a diagram illustrative of aconfiguration for multi-user diversity. The minimum frequency sub-bandused for frequency-dependent scheduling of a UE typically comprisesseveral sub-carriers and may be referred to as a Resource Block (RB)520. Reference number 520 is only pointing to one of the 8 RBs per OFDMsymbol shown as example and for drawing clarity. RB 520 is shown with RBbandwidth 525 (typically comprising of a predetermined number ofsub-carriers) in frequency dimension and time duration 510 (typicallycomprising of a predetermined number of OFDM symbols such as one TTI) intime dimension. Each RB may be comprised of continuous sub-carriers andthus be localized in nature to afford frequency-dependent scheduling(localized scheduling). A high data rate UE may use several RBs withinsame TTI 530. UE #1 is shown as an example of a high rate UE. Low datarate UEs requiring few time-frequency resources may be multiplexedwithin the same RB 540 or, alternatively, the RB size may be selected tobe small enough to accommodate the lowest expected data rate.

Alternatively referring to FIG. 6, which is a diagram illustrative of aconfiguration for frequency diversity, an RB 620 may correspond to anumber of sub-carriers substantially occupying the entire bandwidththereby offering frequency diversity (distributed scheduling). This maybe useful in situations where CQI feedback per RB is not available or itis unreliable (as is the case for high speed UEs) and only CQI over theentire frequency band is meaningful. Therefore, a sub-band (or RB)consists of a set of sub-carriers that may be either consecutive ordispersed over the entire spectrum. It should be noted, that anotheroption to achieve frequency diversity is to assign to a UE two or moreRBs with each RB comprising of contiguous sub-carriers but and with eachRB occupying non-contiguous parts of the bandwidth. In such cases, an RBalways consists of a contiguous set of sub-carriers (for both localizedand distributed scheduling).

By assigning transmission to various simultaneously scheduled UEs indifferent RBs, the Node B scheduler can provide intra-cell orthogonalityamong the various transmitted signals. Moreover, for each individualsignal, the presence of the cyclic prefix provides protection frommultipath propagation and maintains in this manner the signalorthogonality.

Each scheduled UE is informed of its scheduling assignment by theserving Node B through the downlink (DL) control channel. This controlchannel typically carries the scheduled UE identities (IDs), RBassignment information, the MCS used to transmit the data, the transportblock size, and hybrid ARQ (HARQ) information relating to possible datapacket re-transmissions in case of a previous erroneous reception forthe same data packet. The control channel may also optionally carryadditional information such as for the support of a multi-inputmulti-output (MIMO) scheme for transmission and reception with multipleantennas. A scheduling assignment may relate either to data transmissionfrom the Node B to a UE (DL of a communication system) or to datatransmission from a UE to the Node B (UL of a communication system).

According to one prior art method for the control channel transmission,such as the one employed by the WiMax communication system, the controlchannel information for all scheduled UEs is jointly coded with a knownMCS. This MCS has to be a low one in terms of spectral efficiency (forexample, QPSK modulation and low rate convolutional or turbo coding withpossible repetitions) as the control channel needs to be received by allUEs in the serving Node B area including ones potentially experiencingvery low SINR. As a result, the control channel size and correspondingoverhead may become excessively large, thereby adversely affecting thesystem throughput.

According to another prior art method for the control channeltransmission, the control channel information for all scheduled UEs isseparately coded with a known fixed MCS and power control may be appliedto the transmission. In this manner, the control channel transmissionpower for UEs located closer to the serving Node B is reduced while thetransmission power for UEs located near the edge is increased to accountfor the path loss. This scheme improves the overall spectral efficiencyby reducing the interference caused by the control channel transmission.Nevertheless, as power control adaptation is based on prior CQI feedbackfrom UEs, it is not generally possible to predict the futureinterference conditions in order for the power control to be effective.Moreover, this transmission method may result to significant andunpredictable interference variations making the whole schedulingprocess less reliable. For example, a conflict occurs when the controlchannels to cell edge UEs in adjacent Node Bs are transmitted from theseNode Bs using substantially the same frequency resources. Then,transmission power control is ineffective as it is substantiallycancelled since the interfering transmissions apply the sametransmission power control. Separate transmission of the control channelto scheduled UEs with a fixed MCS is used in the 3GPP HSDPAcommunication system.

Thus, there is a need for a method to provide reliable transmission ofthe DL control channel while optimizing the corresponding spectralefficiency of the transmission in a communication system.

SUMMARY

Embodiments of the invention provide methods for robust control channeltransmission with optimum spectral efficiency in the downlink (DL) of acommunication system. Based on explicit or implicit channel qualityindicator (CQI) feedback from user equipments (UEs) and the estimatedpath loss, a serving base station (Node B) transmits the control channelto a scheduled UE using a modulation and coding scheme (MCS)substantially determined from the CQI feedback for said UE. This CQIfeedback may be explicit for the communication in the DL channel orimplicit through the transmission, for example, of a reference signalfor the communication in the uplink (UL) channel. The spectralefficiency of the corresponding control channel transmission isoptimized by having the Node B select the appropriate MCS based on theCQI estimate from scheduled UEs.

As the number of DL and UL scheduled UEs may vary during consecutivetransmission time intervals (TTIs), and the corresponding controlchannels may be potentially transmitted with different MCS, the size ofthe total control channel may also vary. The Node B communicates thenumber of DL and UL scheduling assignments in each MCS region, belongingto a predetermined set of MCS regions, through a field that isseparately transmitted prior to the remaining part of the controlchannel carrying the DL and UL scheduling assignments. Alternatively,the Node B may communicate the size of the control channel in each ofthe MCS regions. This field has a fixed and pre-determined size and MCSand should be received at least by all DL and UL UEs having schedulingassignments and potentially by all UEs communicating with the referenceNode B regardless if they receive a scheduling assignment during thereference TTI. Then, each UE can know of the size of the control channeland, for a pre-determined form of transmission of the control channelcodewords in the various MCS regions (for example, control channelcodewords in the lowest MCS region are transmitted first inpredetermined time-frequency resources, followed by the codewords in thesecond lowest MCS region, and so on) the UE can know of the MCS regionwhere its control channel codeword is transmitted. Since all scheduledUEs can know the control channel size, no time-frequency resources arewasted for transmission of DL data packets since all of the remainingresources can be utilized for data transmission without additionalsignaling, thereby effectively additionally improving the spectralefficiency of the control channel transmission.

These and other features and advantages will be more clearly understoodfrom the following detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosure and the advantagesthereof, reference is now made to the following brief description, takenin connection with the accompanying drawings and detailed description,wherein like reference numerals represent like parts.

FIG. 1 is a diagram illustrative of the Frequency-Time Representation ofan OFDM Signal;

FIG. 2 is a diagram illustrative of using cyclic prefix (CP) toeliminate ISI and perform frequency domain equalization;

FIG. 3 is a diagram illustrative of Cyclic Prefix (CP) Insertion

FIG. 4 shows the concepts of frequency and multi-user diversity;

FIG. 5 is a diagram illustrative of a configuration for Multi-UserDiversity;

FIG. 6 is a diagram illustrative of a configuration for frequencydiversity;

FIG. 7 shows an exemplary cell structure highlighting the cell edgeswhere reserved resource blocks are used for transmission by each Node Bthrough application of interference co-ordination through fractionalfrequency re-use;

FIG. 8 shows an exemplary partitioning of the downlink transmission timeinterval (TTI) illustrating the transmission of the control channelCategory 0, the remaining control channel, and the data channel;

FIG. 9 shows an exemplary transmission of the control channel Category0, and of the remaining control channel in various modulation and codingscheme (MCS) regions. Time division multiplexing (TDM) is assumedbetween the control and data channels; and

FIG. 10 shows an exemplary transmission of the control channel Category0, and of the remaining control channel in various modulation and codingscheme (MCS) regions. Frequency division multiplexing (FDM) is assumedbetween the control and data channels.

DETAILED DESCRIPTION

It should be understood at the outset that although an exemplaryimplementation of one embodiment of the disclosure is illustrated below,the system may be implemented using any number of techniques, whethercurrently known or in existence. The disclosure should in no way belimited to the exemplary implementations, drawings, and techniquesillustrated below, including the exemplary design and implementationillustrated and described herein, but may be modified within the scopeof the appended claims along with their full scope of equivalents.

Embodiments of the invention address the problem of spectrally efficientcontrol signaling design for scheduling of downlink (DL) and uplink (UL)data packet transmissions in OFDMA-based networks, including variants ofthe OFDMA transmission method such as the single-carrier FDMA (SC-FDMA)transmission method. The DL of a communication system refers to thecommunication from a serving base station (also commonly referred to asNode B) to one or more UEs and the UL refers to the communication fromone or more UEs to a serving Node B. The control channel is transmittedin the DL (from the Node B to the scheduled UEs).

The DL control channel, also referred to as DL shared control channel(SCCH), is a major part of the DL overhead (in addition to referencesignals and other synchronization or broadcast channels) that directlyimpacts the achievable throughput and peak data rates. Minimization ofthis overhead requires corresponding minimization of the correspondingsignaling bits and optimization for the spectral efficiency of the SCCHtransmission. While only minor improvements are possible for the former,mainly through efficient mapping techniques for the scheduled UEidentities (IDs) and frequency resource block (RB) allocations, thelatter requires careful transmission design.

In the following discussion, a field indicating the SCCH size and thenumber of DL and UL scheduled UEs is referred to as SCCH Category 0 (orCat0). This field does not carry any information relating to DL or ULscheduling assignments. Rather, its purpose is to dimension the controlchannel so that UEs know how to decode the remaining SCCH that carriesthe scheduling related information. The SCCH part carrying the scheduledUE IDs and allocated RB position for each scheduled UE is referred toSCCH Category 1 (or Cat1). The remaining SCCH part is referred to asCategory 2 (or Cat2). Cat2 carries information related to the modulationand coding scheme (MCS) applied to the data transmission, the transportformat, hybrid ARQ (HARQ) information relating to possible data packetretransmissions and possibly additional information such as for themulti-input multi-output (MIMO) scheme applied to data transmission.

This invention describes an SCCH structure involving Cat0 and theremaining SCCH (carrying scheduling assignment information). Thisinvention also describes a transmission method targeting the spectralefficiency optimization for the remaining SCCH transmission. For the DLscheduling assignments (or scheduling grants), Cat2 may be transmittedeither separately to Cat1 in RBs assigned to DL data transmission ortogether with Cat1 (in a single codeword as known in prior art).Clearly, only joint transmission of Cat1 and Cat2 is possible for ULscheduling assignments as there are no corresponding RBs in which datatransmission follows in the DL (unless a UE has simultaneous DL and ULscheduling assignments). For ease of reference, the remaining SCCH(other than Cat0), will be referred to as Cat1, particularly in someFigures. It is not relevant to the invention whether Cat2 for DLassignments, as described in the previous paragraph, is transmittedtogether or separately to Cat1.

FIG. 8 shows an exemplary partition for Cat0 810 and the remaining SCCH820. As it was previously mentioned, Cat0 has a predetermined size andtransmitted with an MCS that is known to all UEs. It specifies the sizeof the remaining SCCH. It is not necessary that Cat0 is transmitted incontiguous sub-carriers before the remaining SCCH and in practice thesub-carriers carrying the two may be multiplexed to provide frequencydiversity. The remaining time-frequency resources can be assumed to beallocated to data 830, or other channels such as the reference signalchannel, the synchronization channel, and the broadcast channel.

FIG. 9 and FIG. 10 show an exemplary structure for SCCH Cat0 and Cat1(remaining SCCH) further illustrating the embodiments of the invention.Time Division Multiplexing (TDM) of the control and data channels isassumed in FIG. 9. Frequency Division Multiplexing (FDM) is assumed inFIG. 10. In the following, we refer to the TDM option but the samedescriptions and arguments also apply for the FDM one.

Cat0 910 informs the UEs of the remaining SCCH size, thereby limitingthe waste of resources associated with having a fixed SCCH size whichmay not always be filled. The exemplary SCCH granularity in FIG. 9 isone RB 970 in one OFDM symbol 980 but it can generally be any number ofsub-carriers, including one sub-carrier, or even one OFDM symbol.Obviously, the smallest granularity in an MCS region is specified by theminimum number of resources (typically RBs) required for thetransmission of a single DL or UL scheduling assignment, whichever issmaller. Larger granularities than the minimum one may also be used andmay extent to half or even one OFDM symbol. In such cases, the SCCH sizespecified by Cat0 in an MCS region implies that the each of the numbersfor the corresponding DL and UL scheduling assignments is above the onesfor the next lower possible SSCH size, if any, and equal to or smallerthan the ones for the specified SSCH size.

The control channel (scheduling assignment) corresponding to a scheduledUE is transmitted with a MCS determined by the SINR that will beexperienced by the transmission to that UE. The serving Node B candetermine this SINR either based on the DL CQI reported by each UEhaving a DL scheduling assignment, or implicitly based on the UL CQI theserving Node B determines for each UE having an UL schedulingassignment. The larger the SINR, the higher the MCS in terms of spectralefficiency. As the exemplary embodiment considers that the controlchannel transmission from the serving Node B to each scheduled UE isdistributed in frequency, the MCS region may be determined based on theaverage SINR and not the individual SINR in each RB. Three MCS regions920, 930, and 940 are considered (as an example) in FIG. 9 (the sameapplies in FIG. 10) and the remaining RBs in the OFDM symbols of a TTIare allocated to data 950 and other channels, such as for referencesignals (not shown). Reference signals may also occupy OFDM symbolswhere the control channel is transmitted. In general, more than threeMCS regions may be used as shown for example in Table 1, shown below.Repetition coding of one MCS results into a different MCS (with spectralefficiency that is inversely proportional to the repetition factor).

TABLE 1 MCS 7 16QAM, R = ⅔ MCS 6 16QAM, R = ½ MCS 5 16QAM, R = ⅓ MCS 4QPSK, R = ½ MCS 3 QPSK, R = ⅓ MCS 2 QPSK, R = ¼ MCS 1 QPSK, R = ⅓, 2 ×repetition MCS 0 QPSK, R = ¼, 2 × repetition

The lowest MCS region 925 in terms of spectral efficiency (oradditionally the next lower MCS region(s) 935 if possible in terms ofavailable resources) may be associated with reserved RBs for use at thecell edge through the application of cell edge interferenceco-ordination through fractional frequency reuse (IC-FFR), embodimentsof which are described in co-pending U.S. application Ser. No.11/535,867. With IC-FFR, certain RBs in a reference Node B are reservedto be protected by interference from interfering (adjacent) Node Bs byimposing the restriction that the interfering Node Bs do not transmitwith full power in the RBs reserved by the referenced Node B (FIG. 7).In the example of FIG. 7, cell 1 is allocated one-third of that spectrum710, cells 2, 4, and 6 are allocated a second one-third 720, and cells3, 5, and 7 are allocated the final one-third 730. When the Node Bscheduler of any of the previous cells schedules a set of UEs fortransmission, it may assign, for example, the one-third of thesescheduled UEs it determines to be located closer to the cell edge (thanthe remaining two-thirds of UEs) in the one-third of reserved RBs thisreference Node B has been allocated. The remaining two-thirds ofscheduled UEs, deemed to be located closer to the cell interior, arescheduled in the remaining two-thirds of the available spectrum. WithIC-FFR, the low SINR values of the geometry CDF are improved and norepetition coding may be necessary, thereby improving spectralefficiency and avoiding unnecessary increase of the SCCH size and thecorresponding overhead.

As an alternative to IC-FFR, fractional time re-use (IC-FTR) may be usedfor synchronous networks. Embodiments of IC-FTR are described inco-pending U.S. application Ser. No. 11/535,867. With IC-FTR, the entirefrequency bandwidth is always used and the division of resources occursin time. Cell edge UEs having a reference serving Node B are scheduledonly during specific TTIs. For the control channel transmission, thenumber of MCS regions per TTI can be reduced while achieving the gainsfrom link adaptation. This is because only a subset of MCS regions maybe needed during TTIs that cell edge UEs are not scheduled. Anotherimportant side benefit is that cell edge UEs need to monitor the controlchannel only during TTIs where scheduling is possible, thereby enablingsuch UEs, for which battery savings are most important, to go into asleep mode in the remaining TTIs. To enable this sleep mode, cell-edgeUEs may be classified and informed of the TTIs where scheduling ispossible through low rate signaling of the corresponding time schedulingpattern (in the order of tens of seconds—FTR reconfiguration period).

Transmission of the control near the beginning of the TTI (for example,within the first 3-4 OFDM symbols of a 14-symbol TTI) can enable theso-called “micro-sleep” operation for the UE. With micro-sleep, if theUE determines it is not scheduled during a TTI, it may shut down partsof its transmitter and receiver chain for the remaining of the TTI andturn them back on again in time to receive the control channel of thenext TTI. This enables the UE to conserve battery power. However, whenthe UE goes into “micro-sleep” it may miss some of the reference signals(RS) that are significant for improved channel estimation anddemodulation performance of subsequent channels, such as the controlchannel in the next TTI. However, this channel estimation loss dependson the propagation characteristics the channel medium introduces to thetransmitted signal prior to its reception by the reference UE. Thelarger the frequency selectivity of the channel medium, the larger thechannel estimation loss from reducing the number of RS. Therefore, theUE may perform micro-sleep only when it determines that the channelmedium is not very frequency selective (nearly flat channel) as in suchcases channel estimation performance is very accurate even with areduced set of possible RS. Also, as RS time interpolation to previousTTIs is possible for low UE speeds, the UE may perform micro-sleep if itestimates a low Doppler shift implying a low UE speed for a givenoperating carrier frequency.

The main embodiment of this invention, that is further described in theremaining of this application, can be summarized as follows:

-   -   The control channel codeword carrying the scheduling assignment        information for each DL or UL scheduled UE is transmitted with        an MCS corresponding to the SINR conditions of the referenced UE        as determined by the serving Node B. As the SINR conditions        experienced by UEs in the serving area of a Node B may have        significant variations, multiple MCS regions are used to capture        the SINR conditions. The larger the number of MCS regions, the        smaller the granularity of the SINR range captured by each MCS        region. The MCS regions are predetermined.

A field referred to as Cat0 is transmitted by the serving Node B andshould be designed so that it is received by all UEs with a desiredreliability. Cat0 implicitly or explicitly informs the size of theremaining control channel in each MCS region. Cat0 is transmitted by theserving Node B with a predetermined MCS and a predetermined size andboth are known in advance by all UEs. The MCS regions are predetermined.

Additional attributes of the SCCH transmission (including Cat0) areoutlined as follows:

-   -   a) Puncturing and repetition may be used to fit Cat0 or the        remaining SCCH into an integer number of RBs (or, in general for        the remaining SCCH, into a multiple of the minimum number of        resources required for the transmission of a control channel        codeword for DL or UL scheduling assignments in each MCS        region).    -   b) The number of RBs for each MCS region directly depends on the        number of DL and UL scheduled UEs having their SCCH (other than        Cat0) transmitted in that MCS region.    -   c) With application of IC-FFR, at least one or more RBs are        reserved in each cell for protection from inter-cell        interference. The position of the first reserved RB can be a        function of the cell ID or it can be signaled in the        synchronization channel (SCH) or in the broadcast channel (BCH).        Two signaled bits are required for an effective soft frequency        re-use factor of 3 with IC-FFR. SCCH transmission (including        Cat0) to cell edge UEs is carried through the reserved RBs that        are protected from most of the inter-cell interference.    -   d) With IC-FFR, the relative position of the MCS regions can be        specified relative to the one of the reserved RBs.        Alternatively, without IC-FFR, the relative position of the MCS        regions may depend on the order of these regions. For example,        the first RB in FIG. 8 may be occupied by the lowest (UE        populated) MCS region, the second RB may be occupied by the next        lower MCS region and so on. The MCS regions may be ranked in        accordance to their spectral efficiency. For example, an MCS        employing QPSK modulation and code rate of 1/3 has a lower rank        than an MCS region employing QAM16 modulation and code rate 1/3.    -   e) Frequency hopping (FH) is applied to RBs (or sub-carriers)        carrying the same control channel codeword to provide frequency        diversity. This also allows effective link adaptation for        distributed scheduled UEs.    -   f) If one MCS region ends while another continues, the RBs (or        sub-carriers) of the latter can continue following the same        pattern, leaving RBs (or sub-carriers) that would be occupied by        the former for the data channel. Alternatively, they can change        the pattern and occupy RBs (or sub-carriers) of the former so        that Cat1 has a continuous structure (for a small loss in        frequency diversity). This can be predetermined and each UE        knows of the RBs (or sub-carriers) occupied by Cat1 (remaining        SCCH) through Cat0.    -   g) Based at least on the reported CQI, the scheduler first        determines the number of UEs whose SCCH transmission (not        including Cat0) can achieve the desired codeword error rate        target (e.g. 1%) with the highest MCS. Subsequently, the second        highest MCS is considered, and so on until the SCCH of all UEs        selected for scheduling is mapped onto a certain MCS.    -   h) If for any scheduled UEs, the SCCH transmission cannot        achieve the desired target codeword error rate (at the lowest        predetermined MCS region), the transmission may either still        occur if it can achieve reasonably low error rate or scheduling        of these UEs can be postponed for a later transmission time        interval (TTI)-blocked transmission. The selection of the lowest        MCS region should be such that blocked transmissions are very        infrequent (e.g. 1% or less probability of a blocked        transmission) and depends on the SINR distribution of UEs in the        serving Node B.    -   i) The size of each MCS region may vary between consecutive TTIs        depending on the number of UEs whose Cat1 (remaining SCCH) is        transmitted in each MCS region.    -   j) Cat0 and the remaining SCCH are transmitted with priority to        data at or near the beginning of a transmission time interval.

Convolutional or turbo coding is used for the control channeltransmission depending on the codeword size in each MCS region assuggested in co-pending U.S. Application 60/746,450. If DL and ULcontrol channel codewords (not including Cat0) that are transmitted inthe same MCS region are jointly coded, convolutional coding may be usedwhen the number (and total size) of these codewords is small. Turbocoding may be used otherwise. In the case of joint coding, each UEimplicitly determines the coding type (convolutional or turbo) afterdecoding Cat0 and finding the number of DL and UL codewords in each MCSregion.

The invention utilizes the estimate of the SINR that will be experiencedby each transmission from the serving Node B to each DL or UL scheduledUE to code the control channel information (scheduling assignments) inthe appropriate MCS region in order to ensure reception with a targeterror rate. In addition to the SINR, the MCS region depends on thetransmitter and receiver antenna diversity, on the UE speed (asdetermined for example based on Doppler shift estimation at the servingNode B), and on the multi-path propagation conditions introduced by thechannel medium to each transmitted control channel signal as theydirectly impact the achievable control channel codeword error rate for agiven SINR value. For DL scheduled UEs, the SINR is determined from theCQI feedback these UEs provide to the serving Node B in order for thelatter to schedule the transmission of data packets (by determining theMCS and the RBs used for the data transmission to a correspondingscheduled UE). For UL scheduled UEs, the SINR may be determined at theserving Node B through the transmission of a reference signal by each ULscheduled UE over the entire UL scheduling bandwidth for that UE.

Although the DL communication channel used for the control signaling(scheduling assignments) transmission and the UL communication channelused to obtain an SINR estimate for UL scheduled UEs may have differentfading characteristics, the additional diversity provided by thepossible multiple transmitter and receiver antennas and the frequencyhopped transmission of the control channel introducing frequencydiversity can effectively mitigate the impact of variations in thefading characteristics between the two communication channels. Moreover,as the multi-path propagation characteristics experienced by a given UEare typically the same in the DL and UL of a communication system, theNode B may use this information to provide additional protection to UEsexperiencing low multi-path diversity by placing the corresponding ULcontrol signaling information in a lower MCS region than indicated bythe UL SINR measurement, thereby providing some performance margin.

Moreover, as unicast transmission (that is, dedicated communicationbetween a serving Node B and a UE) may be TDM with multicast/broadcasttransmission (that is, transmission of the same information frommultiple Node Bs), DL unicast may not exist during certain TTIs while ULunicast may continue. For this reason, the control channel for unicastmay still be transmitted in multicast/broadcast TTIs carrying only ULscheduling information, as suggested in U.S. Application 60/733,675. Insuch cases, Cat0 carries the aforementioned information only for ULscheduling assignments.

While several embodiments have been provided in the disclosure, itshould be understood that the disclosed systems and methods may beembodied in many other specific forms without departing from the spiritor scope of the disclosure. The examples are to be considered asillustrative and not restrictive, and the intention is not to be limitedto the details given herein, but may be modified within the scope of theappended claims along with their full scope of equivalents. For example,the various elements or components may be combined or integrated inanother system or certain features may be omitted, or not implemented.

Also, techniques, systems, subsystems and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the disclosure. Other itemsshown or discussed as directly coupled or communicating with each othermay be coupled through some interface or device, such that the items mayno longer be considered directly coupled to each other but may still beindirectly coupled and in communication, whether electrically,mechanically, or otherwise with one another. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

1. In a communication system having at least one Node B serving a plurality of user equipments (UEs), a method to transmit control signaling information from said Node B to said UEs during a transmission time interval, wherein each control signal information is transmittable with a Modulation and Coding Scheme (MCS) in a predetermined set of MCS, said method comprising the steps of: said Node B using a first MCS to transmit first control signaling information to a first UE in said plurality of UEs; and said Node B using a second MCS to transmit a second control signaling information to a second UE in said plurality of UEs, wherein said first UE and said second UE do not know said first MCS and said second MCS, respectively, and do not know the location of said first control signaling information and said second control signaling information, respectively, prior to decoding said first control signaling information and said second control signal information, respectively.
 2. The method of claim 1, wherein said Node B transmits said first control signaling information to said first UE and said second control signaling information to said second UE with the same MCS by jointly coding said first control signaling information and said second control signaling information.
 3. The method of claim 1, wherein the location of said first control signaling information transmitted with said first MCS to said first UE is determined based on the location of a resource for said first UE and said first MCS.
 4. The method of claim 1, wherein said first said control signaling information conveys scheduling assignments to a UE for data transmission from said Node B to said UE and said second said control signaling information conveys scheduling assignments to same said UE for data transmission from said UE to said Node B, said first control signaling information and said second control signaling information being jointly coded to produce a joint control signaling information.
 5. The method of claim 1, wherein said Node B determines said modulation and coding scheme to transmit said control channel information to at least one of said UEs based on a channel quality estimate obtained by said Node B from said at least one of said UEs.
 6. The method of claim 5, wherein said channel quality estimate is explicitly transmitted from said at least one of said UEs to said Node B.
 7. The method of claim 5, wherein said channel quality estimate is determined at said Node B based on a signal transmitted from said at least one of said UEs to said Node B.
 8. The method of claim 1, wherein said Node B determines said modulation and coding scheme based on a target reception error rate of said control signaling information for at least one of said UEs and said Node B transmits said control signaling information with the modulation and coding scheme of the highest spectral efficiency satisfying said target reception error rate at said at least one of said UEs.
 9. The method of claim 8, wherein said modulation and coding scheme of the highest spectral efficiency is from a predetermined set of multiple modulation and coding schemes.
 10. The method of claim 1, wherein said Node B determines said modulation and coding scheme to transmit said control channel information based on the number of said Node B transmitter antennas.
 11. The method of claim 1, wherein said Node B determines said modulation and coding scheme to transmit said control channel information to at least one of said UEs based on the number of receiver antennas for said at least one of said UEs.
 12. The method of claim 1, wherein said Node B determines said modulation and coding scheme to transmit said control channel information to at least one of said UEs based on multi-path propagation characteristics introduced to the signal transmission by the channel medium.
 13. The method of claim 12, wherein said Node B determines said multi-path propagation characteristics based on a signal transmitted by said at least one of said UEs to said Node B.
 14. The method of claim 1, wherein said Node B determines said modulation and coding scheme to transmit said control channel information to at least one of said UEs based on an estimate for the velocity for said at least one of said UEs.
 15. The method of claim 1, wherein said Node B transmits said first control channel information using frequency hopping.
 16. The method of claim 1, wherein said Node B transmits said control channel information with priority to data.
 17. The method of claim 1, wherein said communication system employs the OFDMA transmission method.
 18. In a communication system having at least one Node B serving a plurality of user equipments (UEs), a method to transmit control signaling information from said Node B to said UEs during a transmission time interval comprising the steps of: said Node B using one modulation and coding scheme to transmit said control signaling information to at least a first UE in said plurality of UEs; said Node B using a second modulation and coding scheme, which is different than said first modulation and coding scheme, to transmit said control signaling information to at least a second UE in said plurality of UEs, which is different than said first UE of said plurality of UEs; and wherein said Node B transmits a first said control signaling information to a first UE and a second said control signaling information to a second UE with the same said modulation and coding scheme by jointly coding said first control signaling information and second said control signaling information to produce a joint control signaling information.
 19. The method of claim 18, wherein said Node B transmits said joint control signaling information using a first coding scheme for a first size of said joint control signaling information and using a second coding scheme for a second size of said joint control signaling information.
 20. The method of claim 19, wherein said first coding scheme is convolutional coding and said second coding scheme is turbo coding.
 21. In a communication system having at least one Node B serving a plurality of user equipments (UEs), a method to transmit control signaling information from said Node B to said UEs during a transmission time interval comprising the steps of: said Node B using one modulation and coding scheme to transmit said control signaling information to at least a first UE in said plurality of UEs; said Node B using a second modulation and coding scheme, which is different than said first modulation and coding scheme, to transmit said control signaling information to at least a second UE in said plurality of UEs, which is different than said first UE of said plurality of UEs; wherein a first said control signaling information conveys scheduling assignments to a UE for data transmission from said Node B to said UE and a second said control signaling information conveys scheduling assignments to same said UE for data transmission from said UE to said Node B, said first control signaling information and said second control signaling information being jointly coded to produce a joint control signaling information; and wherein said Node B transmits said joint control signaling information using a first coding scheme for a first size of said joint control signaling information and using a second coding scheme for a second size of said joint control signaling information.
 22. The method of claim 21, wherein said first coding scheme is convolutional coding and said second coding scheme is turbo coding.
 23. In a communication system having at least one Node B serving a plurality of user equipments (UEs). a method to transmit control signaling information from said Node B to said UEs during a transmission time interval comprising the steps of: said Node B using one modulation and coding scheme to transmit said control signaling information to at least a first UE in said plurality of UEs; and said Node B using a second modulation and coding scheme, which is different than said first modulation and coding scheme, to transmit said control signaling information to at least a second UE in said plurality of UEs, which is different than said first UE of said plurality of UEs; and wherein the location of said control signaling information transmitted with a said modulation and coding scheme is determined based on the location of a reserved resource and the order of said modulation and coding scheme in a set of multiple modulation and coding schemes. 